In EEPROM memories, the logical value of a bit stored in a memory location is represented by the value of the threshold voltage of a floating gate transistor, which may be modified at will by write operations. A write operation generally comprises an erase step followed by a programming step.
However, in certain cases, the write operation may comprise only an erase step or only a programming step. Thus, if, for example, the word to be written only contains “o”s, then only an erase step is needed. If the previous content of the memory location in which it is desired to write a digital word already contains only “o”s, then the erase step is unnecessary.
The programming or the erasing of a floating gate transistor consists of the injection or the extraction of electrical charges into/from the floating gate of the transistor by tunnel effect (“Fowler-Nordheim effect”) through the gate oxide called “tunnel oxide”, by means of a high voltage pulse Vpp which can be of the order of 10 to 20 volts, typically 13 volts.
This high voltage of 13 volts needed for the writing of EEPROM memories is non-reducible and is very constraining with regard to the technological processes and the reliability of the product.
Indeed, lithographic reduction, in other words an increase in the etch resolution, leads to a decrease in the operating voltages, and this high write voltage becomes more problematic with regard notably to leakages from the source/drain junctions of the transistors and also to breakdown of the tunnel oxides.
Consequently, these risks of breakdown and of premature aging of the transistors have a direct impact on the reliability of the product and the maximum high voltage Vpp applicable is limited by the robustness of the memory cells.
Furthermore, when the voltage Vpp comes close to the maximum permitted voltages for the components in question, high leakage currents appear, generally by avalanche effect. These currents increase dramatically above a certain threshold and a charge pump can no longer supply them. This can lead to an under-erasing or an under-programming, and these risks of leakages thus have a direct impact on the functionality of the circuit.
The electric field needed to obtain a tunnel current by “Fowler-Nordheim effect” is notably proportional to the applied voltage Vpp, to the drain-floating gate coupling factor and to the inverse of the thickness of the layer of tunnel oxide.
Maximizing the coupling factor of the memory cells and minimizing the thickness of the tunnel oxide have provided a partial solution to this problem, but these techniques have attained their maximum possibilities (coupling factor exceeding 80% and thickness of tunnel oxide less than 70 Å).
An increase in the duration of application of the erase and programming pulses is limited because it can lead to unacceptable write times.
Consequently, it is in particular the write problems that pose a barrier to the development of modern technologies of non-volatile memories of the EEPROM type.
Furthermore, there exists a need for low power operation of memories, and hence to limit the value of the voltages implemented, notably for autonomous systems powered by small batteries, such as hearing aids, or for radiofrequency identification “RFID” tags.
Thus, it is desirable to reduce this high voltage Vpp, while at the same time ensuring a reliable and efficient writing of the data into the memory locations.